Combined electrodialysis and ultrafiltration of an electrodeposition bath

ABSTRACT

A METHOD FO CONTROLLING UNDESIRABLE COMPONENTS IN AN ELECTROPEDPOSTION BATH COMPRISING A FILM-FORMING VEHICLE RESIN IN AN AQUEOUS MEDIUM, BY SUBJECTING AT LEAST A PORTION OF THE ELECTRODEPOSITABLE COMPOSITION, SIMULTANEOUSLY OR AT A SEPARATE TIME, TO AN ELECTRODIALYSIS PROCESS IN CONJUCTION WITH SELECTIVE SEPARATION PROCESS WHEREBY PROCESS PARAMETERS AND THE PROPERTIES OF THE DEPOSITED FILM ARE MAINTAINED.

3,663,406 ATION May 16, 1972 R. LE BRAS ETAL EGTRODIALYSI OF AN ELECT COMBINED EL S AND ULTRAF'ILTR RODEPOSITION BATH 2 Sheets-Sheet 1 Flled March 11, 1971 FIG. 2

INVENTOR6 w MM m m m 52 /-.M- r W United States Patent US. Cl. 204--181 13 Claims ABSTRACT OF THE DISCLOSURE A method of controlling undesirable components in an electrodeposition bath comprising a film-forming vehicle resin in an aqueous medium, by subjecting at least a portion of the electrodepositable composition, simultaneously or at a separate time, to an electrodialysis process in conjunction with a selective separation process whereby process parameters and the properties of the deposited film are maintained.

BACKGROUND OF THE INVENTION Electrodeposition has recently received wide industrial acceptance as a method for applying protective and decorative coatings. The process of electrodepositing is well described in the art. Generally, an aqueous bath containing depositable coating composition is placed in contact with an electrically-conductive anode and an electricallyconductive cathode, and upon the passage of electric current (usually direct current) between the anode and cathode while in contact with the bath containing the coating composition, an adherent film of the coating composition is deposited, either on the anode or the cathode, depending upon thetype of resin employed. The electrodeposition process parameters used vary widely. The voltage applied may vary from as low as, for example, one volt, or as high as, for example, 500 volts or higher. However, typically the voltage used ranges from 50 to 400 volts. The current demands are higher during the initial stage of the deposition process, but decrease as the deposited film insulates the particular conductive electrode. Generally, the electrode employed may be any electrically-conductive surface, such as iron, steel, aluminum, zinc, copper, chromium, magnesium, galvanized steel, phosphatized steel, as well as other metals and pretreated metals. In some instances, paper or other non-conductive materials can be coated or be impregnated with conductive substances and utilized as the electrode on which electrodepositable compositions are deposited.

A wide variety of electrodepositable resins are known in the art. For example, a number of water-soluble, waterdispersible, or water-emulsifiable polycarboxylic acid resins can be electrodeposited. Some of these resins include: reaction products or adducts of a drying oil or semidrying oil fatty acid with a dicarboxylic acid or anhydride; interpolymers of a hydroxyalkyl ester of an unsaturated carboxylic acid and an unsaturated monomer; alkydamine vehicles; that is, vehicles containing an alkyd resin and an amine-aldehyde resin; and mixed esters of resinous polyols. The electrodepositability of various other materials, including a number of waxes and natural and synthetic resins, are also known in the art.

In the past, great difliculty has been experienced in maintaining bath stability. That is, after continued bath usage and aging, solubilizing agent and other undesirable impurities and contaminants tend to accumulate in the bath. Such undesirable components deleteriously affect the coating process; for example, the voltage at which the deposited film rupturesdrops significantly, conductivity "ice of the bath increases, pH of the bath increases, and film thickness decreases, and the chemical and physical properties of the deposited film become unsatisfactory.

It is known that bath stability may be improved by employing electrodialysis. By means of electrodialysis, undesirable contaminants and impurities which have a charge opposite the particular membrane-enclosed electrode are accelerated by an electromotive force through a semi-permeable membrane and thereby expelled from the system. In addition to electrodialysis, selective separation processes such as ultra-filtration have been utilized to purge electrodepositable compositions of deleterious material. Although ultrafiltration is sufiicient to control the undesirable contents of certain bath compositions, in some cases ultrafiltration alone is not suflicient to maintain a uniform product. For example, when potassium hydroxide is employed in the solubilization of an anionic vehicle, the potassium ions which accumulate during the electrodeposition process in the coating composition cannot in many instances be removed in sufiicient quantity by employing ultrafiltration.

Each of these systems, moreover, have certain disadvantages. Electrodialysis, while effective in removing counter-ions (ions of the opposite charge), does not remove other ions, many of which are highly undesirable, and does not remove other, uncharged low molecular weight species. Moreover, ions with slow diffusion rates or slow ion mobility may not be removed by electrodialysis at a rate which will enable optimum control of the operating parameter, and in some instances ions having a charge opposite to those ions sought to be removed by electrodialysis process will remain in the electrodepositable composition producing deleterious effects on the coating parameters and deposited coating. Ultrafiltration, on the other hand, removes all materials below a certain molecular size and when used sufficiently to remove all undesirable components tends also to remove some desirable ones.

DESCRIPTION OF THE INVENTION Now it has been discovered that electrodepositable compositions that are not readily susceptible to complete control by either electrodialysis or ultrafiltration can be efficiently controlled by employing a combination of electrodialysis and a selective separation process such as ultrafiltration. By subjecting the electrodepositable composition to both electrodialysis and a selective separation process, virtually all ions, contaminants, and impurities can be purged from the system without substantially removing desired constituents.

Generally, dialysis is the separation of solutes by means of their unequal diffusion rate through membranes, while in electrodialysis the passage of electrolyte through the membranes is accelerated by an electromoti e force. The membranes employed in electrodialysis are frequently referred to as semi-permeable membranes and include various components which are interposed between two bodies of liquid so as to prevent their gross inter-mixture 'but which permit the passage of solvent and at least one of several solutes from one body of liquid to the other.

Electrodialysis is controlled by electromotive force, diffusion rate and the membrane properties. The electromotive force may be the same as that used in the electrodeposition process, however, in many instances a different electromotive force is desirable. Diffusion is the force that drives the molecules and ions toward and when possible across the membranes. The nature of the membrane determines which molecular species can pass and which are held back. Thus, preparation and selection of suitable membranes is of particular importance. A variety of membranes may be employed in the electrodialysis as used in the present invention. These include conventional dialysis membranes such as regenerated cellulose on fabrics or felts; films of polyvinyl compounds as well as membrane materials which are not usually considered as dialysis membranes, but which produce the desired electrodialysis when employed in the electrodeposition process. Among the useful membranes are those comprised of woven or unwoven cloth, including cloth of various natural or synthetic fibers. Other membranes such as cloth treated with re-agents that dissolve cellulose-ammoniacal cupric salt solution or NaOH and carbon disulfide; protein for insolubilized gelatin-soybean protein and animal skin; inorganic membranes from precipitated silica; and ion-exchange membranes are also useful.

Positively-charged membranes are selective to anions and impervious to cations, while negatively-charged membranes are selective to cations and impervious to anions. The choice of membrane depends in part upon the type of electrodepositable composition treated.

The preferred membranes for use in the electrodialysis step of the invention are those made of a plain Woven cloth comprising plant fibers, and selectively permeable ion-exchange membranes.

The membranes comprising plant fibers are advantageous in that they have good durability for industrial use and the investment expense is low, while still having a remarkable degree of selectivity in their permeation characteristics. The plant fibers of the membrane may be fruit fibers, for example, coconut and the like; the fibers may be leaf fibers, for example, Manila hemp, New Zealand flax and the like; the fibers may be vegetable fibers, for example, seed fibers such as cottonseed and the like; and the fibers may be phloem fibers such as flax, linen, hemp, China grass, ramie, jute and the like. The plant fiber membranes need not necessarily be constructed of natural fibers, for example, synthetic fibers having high tensile strength may readily be interwoven with the plant fibers, thus adding strength and durability to the membrane. For a more complete discussion of plant fiber membranes, see US. Pat. No. 3,496,083.

When a plant fiber membrane is immersed in an electrodepositable composition, the fibers generally swell in a direction that is perpendicular to the lengths of the fibers so that the swollen fibers produce a closely woven cloth, thus the pigment and vehicle portions of the electrodepositable composition are not readily passable through the cloth under such conditions. The plant fibers show a negative charge and thus are readily employed as an electrolytically negative diaphragm, that is, a diaphragm membrane that is permeable to cations and impervious to anions.

Selectively permeable ion exchange membranes are also desirably employed in a manner similar to that of the plant fiber membranes. Using an ion-exchange membrane has advantages over using an unselective dialysis mem brane to separate accumulated ions from the depositable compositions, in that the ion-exchange membranes normally have a lower electrical resistance than dialysis membranes, and thus being selective, provide for better control of the pH in the coating bath. Controlling the pH is of particular importance when the coating composition is in use, for in such case, the ion selective membrane permits a faster and more efficient passage of ions of opposite charge through it. Also, the resistivity of the receiving solvent, in contact with the electrode and confined by the membrane, can readily be reduced by the addition of a suitable ionizable material, such as soda ash, ammonium sulphate, sodium sulphate and sodium bicarbonate, without risk of contamination of the coating composition. In some cases, KOH solutions or amide solutions are used.

The ion-exchange membranes employed in this invention may be prepared by the incorporation of finely divided ion-exchange resins in inert polymer matrices. Examples of such resins are fine beads of sulphonated crosslinked'polystyrene in polyethylene, films produced from styreneldivinyl benzene copolymers, when subjected to such treatments as sulphonation to yield cation-exchanges or chloromethylation and amination to yield anion-exchangers, and films of graft copolymers comprising an inert backbone and a reactive grafted component such as styrene.

The ion-exchange membranes employed generally have a pore size of less than 20A, for example, about 10 to 15A. Also, the fixed ion concentration of the membrane is usually at least one unit on the molarity scale, so that if the external concentration is not very high, they conduct almost exclusively by the migration of counter-ions. Typical ion-exchange membranes have sodium ion transport numbers of at least 0.8 or greater, in sodium chloride solutions of IM concentration. For a more complete description of the ion-exchange utilization in electrodialysis, see U.S. Pat. No. 3,419,488.

When an anodic or anionic coating composition is employed, the negatively-charged vehicle will migrate under an electromotive force to the anode where the solubilizing agent is released, and thus positive-charged cations will migrate through the membrane. The reverse is true when a cathodic or cationic coating composition is employed. For example, if potassium hydroxide is utilized as a solubilizing agent in formulation of a depositable composition, the potassium ion released during the electrocoating process, and which tends to accumulate in the anode compartment, is removed by passage into a cathode compartment which is separated from the aqueous electrodepositable composition in the anode compartment by the membrane. When an acid-solubilized cationic resin, such as an amine-terminated polyamide or acrylic polymer is used, the acidic agent tends to accumulate in the anode compartment and is removed in a similar manner.

The electrode compartment used in electrodialysis to separate the anode or cathode from the electrodepositable composition can be of any convenient shape. Perforated cylindrical-shaped plastic containers having the membrane mounted on such super-structure and the electrode encased therein have been employed. However, more commonly utilized structures are rectangular-shaped boxes having the electrode centrally located and the major walls of the said box parallel to the electrode and comprised of the membrane. Usually, the electrode compartment is equipped with an input and outlet connections to facilitate flushing the compartment. The electrode compartment may contain a receiving solvent (electrolyte) comprised of the aqueous depositable composition, but it is preferable to employ water in the electrode compartment, and, in particular, deionized water containing a minimum level of the electrolyte that is being removed. To prevent an accumullation of ions, means for periodic or continuous flushing of the electrode compartment with deionized water or a mixture of deionized Water and a minimum level of the separable electrolyte is preferably provided. In some instances it is possible to fabricate the plant fiber membranes into a bag and by so doing avoid leaks and produce a suitable unit having a low cost construction.

When the membrane employed does not ordinarily act as a dialysis membrane in the absence of a potential between the electrodes (as during shutdown periods), there is little tendency for accumulated ions to rediffuse through the membrane unless the electrolyte level in the electrode compartment is relatively high. Any such tendency can be corrected by continual flushing of the membrane-enclosed compartment or by employing a membrane that is impermeable to water, and also by maintaining a low electrolyte level in the membrane-enclosed compartment.

A cathode compartment (or if a cationic vehicle is employed, the anode compartment) need not be constructed for all the electrode plates, but the surface area of an exposed membrane may be varied depending on the degree of control desirable and also surface area of the article coated. When anionic vehicles are employed, other alkaline anions may be purged from the anode compartment, for example, ammonia, organic amines, sodium ions and the like.

As mentioned above, while electrodialysis alone provides a useful degree of control of electrocoating bath compositions, it is not a complete solution to bath control. Ions of a charge opposite that of the membrane-encased electrode still accumulate and cause undesirable effects on coating parameters and properties of the deposited coating. In potassium hydroxide solubilized systems, the removal of the potassium ion by electrodialysis using an ion exchange membrane may be satisfactory, but the buildup of anions and non-ionic ingredients may eventually cause an imbalance in the bath. In many cases, and particularly where plant fiber membranes are employed, the efiiciency of potassium ion removal is usually not sufiicient and a buildup of the potassium ion results, causing an inbalance in the bath. These problems are overcome by the present invention by utilizing electrodialysis in conjunction with the selective separation process, such as ultrafiltration, which removes the accumulated ions and excess water from the dialyzed composition.

The selective filtration process employed in the process of the instant invention is any process which separates water from the electrodeposition bath through a physical barrier while retaining the solubilized resin components. Thus, any means may be utilized which accomplishes this purpose. Means may pass not only water but also solute of substantially lower molecular weight than the vehicle resin such as excess amine, carbonates, low molecular weight solvent and simple organic or inorganic anions and cations which may be present in the bath. Examples of means for accomplishing this separation are reverse osmosis, where water of high purity may be obtained, and ultrafiltration, which is especially preferred.

In the ultrafiltration process, exceptional control of a bath composition and removal of objectionable accumulated materials is achieved by selectively removing low molecular weight materials from the bath composition. This selective filtration process removes excess counterions and thus serves as a method of conventional bath control, but in addition, this method further removes other excess materials or contaminants from the bath.

Ultrafiltration separates materials below a given molecular weight size from the electrodeposition bath. With properly selected membranes, this treatment does not remove in substantial amounts any product or desirable resin from the paint in the tank, but does remove anionic, cationic and nonionic materials from the paint in a ratio proportional to their concentration in the water phase of the paint. Thus, for example, it is possible to remove amines, alkaline metal ions, phosphates, chromates, sulfates, solvents and dissolved carbon dioxide, among others.

Ultrafiltration may be defined as a method of concentrating solute while removing solvent, or selectively removing solvent and low-molecular weight solute from a significantly higher molecular weight solute. From another aspect, it is a process of separation whereby a solution containing a solute of molecular dimensions significantly greater than the solvent is depleted of solute by being forced under a hydraulic pressure gradient to flow through a suitable membrane. The first definition is the one most fittingly describes the term ultrafiltration as applied to an electrodeposition bath.

Ultrafiltration thus encompasses all membrane-moderated, pressure-activated separations wherein solvent or solvent and smaller molecules are separated from modest molecular weight macromolecules and colloids. The term ultrafiltration is generally broadly limited to describing separations involving solutes of molecular dimensions greater than about ten solvent molecular diameters and below the limit of resolution of the optical microscope, that is, about 0.5 micron. In the present process, water is considered the solvent.

The principles of ultrafiltration and filters are discussed in a chapter entitled Ultrafiltration in the Spring, 1968, volume of Advances in Separations and Purifications, E. S. Perry, editor, John Wiley & Sons, New York, as well as in Chemical Engineering Progress, vol. 64, December 1968, pages 31 through 43, which are hereby incorporated by reference. I

The basic ultrafiltration process is relatively simple. Solution to be ultrafiltered is confined under pressure, utilizing, for example, either a compressed gas or liquid pump in a cell, in contact with an appropriate filtration membrane supported on a porous support. Any membrane or filter having chemical integrity to the system being separated and having the desired separation characteristic may be employed. Preferably, the contents of the cell should be subjected to at least moderate agitation to avoid accumulation of the retained solute on the membrane surface with the attendant binding of the membrane. Ultrafiltrate is continually produced and collected until the retained solute concentration in the cell solution reaches the desired level, or the desired amount of solvent or solvent plus dissolved low molecular weight solute is removed. A suitable apparatus for conducting ultrafiltration is described in US. Pat. No. 3,495,465, which is hereby incorporated by reference.

There are two types of ultrafiltration membranes. One is the microporous ultrafilter, which is a filter in the traditional sense, that is, a rigid, highly-voided structure containing interconnected random pores of extremely small average size. Through such a structure, solvent (in the case of electrodeposition, water) flows essentially viscously under a hydraulic pressure gradient, the flow rate proportional to the pressure difierence, dissolved solutes, to the extent that their hydrated molecule dimensions are smaller than the smallest pores within the structure, will pass through, little impeded by the matrix. Larger size molecules, on the other hand, will become trapped therein or upon the external surface of the membrane and will thereby be retained. Since the microporous ultrafilters are inherently susceptible to internal plugging or fouling by solute molecules Whose dimensions lie within the pore size distribution of the filter, it is preferred to employ for a specific solute a microporous ultrafilter whose mean pore size is significantly smaller than the dimensions of the solute particle being retained.

In contrast, the diffusive ultrafilter is a gel membrane through which both solvent and solutes are transported by molecular diffusion under the action of a concentration of activity gradient. In such a structure, solute and solvent migration occurs via random thermal movements of molecules within and between the chain segments comprising the polymer network. Membranes prepared from highly hydrophilic polymers which swell to eliminate standard water are the most useful dilfusive aqueous ultrafilter membranes. Since a diffusive ultrafilter contains no pores in the conventional sense and since concentration within the membrane of any solute retained by the membrane is low and time-independent, such a filter is not plugged by retained solute, that is, there is no decline in solvent permeability with time at a constant pressure. This property is particularly important for a continuous concentration or separation operation. Both types of filters are known in the art.

The presently preferred ultrafilter is an anisotropic membrane structure such as illustrated in FIG. 1. This structure consists of an extremely thin, about one-tenth to about ten micron layer, of a homogeneous polymer 1 supported upon a thicker layer of a microporous opencelled sponge 2, that is, a layer of about 20 microns to about 1 millimeter, although this dimension is not critical. If desired, this membrane can be further supported by a fibrous sheet, for example, paper, to provide greater strength and durability. These membranes are used with a thin film or skin side exposed to the high pressure solution. The support provided to the skin by the spongy substrate is adequate to prevent film rupture.

Membranes useful in the process are items of commerce and can be obtained by several methods. One general method is described in Belgian Pat. No. 721,058. This patent describes a process which, in summary, comprises (a) forming a casting dope of the polymer in an organic solvent, (b) forming a film of the casting dope, and (c) preferentially contacting one side of said film with a diluent having high compatibility with the casting dope to effect precipitation of the polymer immediately upon coating the cast film with the diluent.

The choice of specific chemical composition for the membrane is determined to a large extent by its resistance to the chemical environment. Membranes can be typically prepared from thermoplastic polymers such as polyvinyl chloride, polyacrylonitrile, polysulfones, poly (methyl methacrylate), polycarbonates, poly(n-butyl methacrylate), as well as a large group of copolymers formed from any of the monomeric units of the above polymers, including Polymer 360, a polysulfone copolymer. Cellulosic materials such as cellulose acetate may also be employed as membrane polymers.

Some examples of specific anisotropic membranes perable in the process of the invention include Diafiow membrane ultrafilter PM-30, the membrane chemical composition of which is a polysulfone copolymer. Polymer 360, and which has the following permeability characteristics The membrane is chemically-resistant to acids (HCl, H 80 H PO all concentrates), alkalis, high phosphate buffer and solutions of common salts as Well as concentrated urea and guanadine hydrochloride. The membrane is solvent-resistant to alcohol, acetone and dioxane. The membrane is not solvent-resistant to dimethylformamide or dimethyl sulfoxide. This membrane is hereinafter referred to as Membrane A.

Dorr-Oliver XPA membrane, the membrane chemical composition of which is Dynel (an acrylonitrile-vinyl chloride copolymer) and which has the following permeability characteristics:

Flux (gaL/sq.

ft.lday at 30 Molecular Percent p.s.i., 1.0%

Solute weight retention solute) Cytochrome C 12, 600 50 100 a-Chymotripsinagen 24, 000 90 22 Ovalbumin 45, 000 100 4,5

This membrane is hereinafter referred to as Membrane B.

Dorr-Oliver BPA type membrane, the membrane chemical composition of which is phenoxy resin (polyhydroxyether), and which has the following permeability characteristics:

This membrane is hereinafter referred to as Membrane C.

The microporous ultrafilters are generally isotropic structures, thus flow and retention properties are independent of flow direction. It is preferred to use an ultrafilter which is anisotropic in its microporous membrane structure, FIG. 2. In such a membrane, the pore size increases rapidly from one face to the other. When the finetextured side 4 is used in contact with the feed solution, this filter is less susceptible to plugging since a particle which penetrates the topmost layer cannot become trapped in the membrane because of the larger pore size 5 in the substrate.

The process of selective separation may be operated continuously or intermittently. In batch selective filtration or batch ultrafiltration a finite amount of material is placed in a cell which is pressurized. A solvent and lower molecular weight solutes are passed through the membrane. Agitation is provided by a stirrer, for example, a magnetic stirrer. Obviously, this system is best used for small batches of material. In a process requiring continuous separation, a continuous selective filtration process is preferred. Using this technique, material is continuously recirculated under pressure over a membrane or series of membranes through interconnecting flow channels, for example, spiral flow channels.

Likewise, the ultrafiltration process may be conducted as either a concentration process or a diafiltration process. Concentration involves removing solvent and low molecular weight solute from an increasingly concentrated retentate. Filtration flow rate -'will decrease as the viscosity of the concentrate increases. Diafiltration, on the other hand, is a constant volume process whereby the starting material is connected to a reservoir of pure solvent, both of which are placed under pressure simultaneously. Once filtration begins, the pressure source is shut off in the filtration cell and thus, as the filtrate is removed, an equal volume of new solvent is introduced into the filtration cell to maintain the pressure balance. The configuration of the filter may also vary widely and is not limiting to the operation of the process. The filter or membrane may, for example, be in the form of a sheet, tubes, or hollow fiber bundles, among other configurations.

Under ideal conditions, selected low molecular Weight solutes would be filtered as readily as solvent and their concentration in the filtrate is equal to that in the retentate. Thus, for example, if a material is concentrated to equal volumes of filtrate and retentate, the concentration of low molecular weight solute in each would be the same.

Using diafiltration, retentate solute concentration is not constant and the mathematical relationship is as follows:

ELL ci V.,

Where C is the initial solute concentration, C, is the final solute concentration of the retentate, V is the volume of solute delivered to the cell (or the volume of the filtrate collected), and V, is the initial solution volume (which remains constant). A

The ultrafiltration process employing a difiusive membrane ultrafilter retains the solubilized vehicle resin while passing water and low molecular weight solute, especially those with a molecular weight below about 500. As previously indicated, the filters discriminate as to molecular size rather than actual molecular weight, thus, these molecular weights merely establish an order of magnitude rather than a distinct molecular weight cut-off. Also, the particular charge on the low molecular weight solutes and ions that pass through the ultrafilter membrane is of little importance since there is no E.M.F. across the ultrafilter membrane, as contrasted with the electrodialysis membrane. Likewise, as previously indicated, the retained solutes may, in fact, be colloidal dispersions or molecular dispersions rather than true solutes.

In the prment process, a portion of the contents of the coating zone is continuously or intermittently passed, usually under pressure created by a pressurized gas or by means of pressure applied to the contained fluid, into contact with the ultrafilter. Obviously, if desired, the egress side of the filter may be maintained at a reduced pressure to create the pressure difference.

The pressures necessary are not severe. The maximum pressure, in part, depends on the strength of the filter. The minimum pressure is that pressure required to force water and low molecular weight solute through the filter at a measurable rate. With the presently preferred membranes, the operating pressures are between about and 150 p.s.i., preferably between about and 75 psi. Under most circumstances, the ultrafilter should have a minimum initial flux rate, measured with the composition to be treated of at least about 3 gals/sq. ft./day (24 hours) and preferably at least about 4.5 gaL/sq. ft./day.

As previously indicated, the bath composition should be in motion at the face of the filter to prevent the retained solute from impeding the fiow through the filter. This may be accomplished by mechanized stirring or by fluid flow with a force vector to the filter surface.

The retained solutes comprising the vehicle resin and pigment can be returned to the electrodeposition bath. If desired, the concentrate may be reconstituted by the addition of water either before entry to the bath or by adding water directly to the bath. The ultrafiltrate may readily be utilized to rinse the paint dragout back into the electrocoating bath or, if desirable, the rinsings may be passed to the drain.

If there is present in the bath desirable materials which, because of their molecular size, are removed in the ultrafiltration process, these may likewise be returned to the bath either diretcly to the retained solute before entry to the bath, in the makeup feed as required, or independently.

A number of electrodepositable resins are known and can be employed to provide the electrodepositable compositions which may be utilized within the scope of ultrafiltration. Virtually any water-soluble, water-dispersible or water-emulsifiable vehicle resin in an aqueous medium can be electrodeposited and, if film-forming, provides coatings which may be suitable for certain purposes. Preferably the resins are ionically-solubilized synthetic resins. The present invention is applicable to any 'such process.

Among the common electrodepositable compositions are those based upon polycarboxylic acid resins. In order to produce an electrodepositable composition from such polycarboxylic acid resins, it is necessary to at least partially neutralize the acid groups present with a base in order to disperse the resin in the aqueous electrodeposition bath. Inorganic bases such as metal hydroxides, especially potassium hydroxide, can be used, as can ammonia or organic bases such as amines. Watersoluble amines are often preferred. Commonly used amines include ethylamine, diethylamine, triethylamine, diethanolamine, and the like.

Other base-solubilized polyacids which may be employed as electrodeposition vehicles include those taught in US. Pat. No. 3,392,165, which is incorporated herein by reference, wherein the acid groups rather than being solely polycarboxylic acid groups contain mineral acid groups such as phosphonic, sulfonic, sulfate and phosphate groups.

The process of the instant invention is also applicable to cationic type vehicle resins, that is, vehicle resins which deposit on the cathode. These include polybases solubilized by means of an acid, for example, an amine-terminated polyamine or an acrylic polymer solubilized with acetic acid. Other cationic polymers include reaction products of polyepoxides with amino-substituted boron esters and reaction products of polyepoxides with hydroxyl or carboxyl-containing amines.

In addition to the vehicle resin, there may be present in the electrodepositable composition any desired pigment or pigment composition, including practically any of the conventional types of pigments employed in the art. Sometimes there is incorporated into the pigment composition a dispersing or surface-active agent. Usually the pigment and surface-active agent, if any, are ground together in a portion of the vehicle, or alone in an aqueuos medium, to make a paste and this is blended with the vehicle to produce a coating composition.

In many instances, it is preferred to add to the electrodeposition bath certain additives to aid dispersibility, viscosity and/or film quality, such as a non-ionic modifier or solvent. There may also be included additives such as antioxidants, wetting agents, anti-foaming agents, fungicides, bactericides and the like.

In formulating the coating composition, ordinary tap water may be employed, but where such water contains a relatively high level of metals and cations, deionized water, i.e., water from which free ions have been removed by the passage through ion exchange resins, is preferably employed.

Electrodepositable compositions, while referred to as solubilized, in fact are considered a complex solution, dispersion, suspension or combination of one or more of these classes, in water, which acts as an electrolyte, under the influence of an electric current. While, no doubt, in some circumstances the vehicle resin is in solution, it is clear that in some instances and perhaps in most the vehicle resin is a dispersion which may be called a molelcular dispersion of molecular size between a colloidal suspension and a true solution.

The typical industrial electrodepositable composition also contains pigments, crosslinking resins and other adjuvants which are frequently combined with the vehicle resin in a chemical and a physical relationship. For example, the pigments are usually ground in a resin medium and are thus wetted with the vehicle resin. As can be readily appreciated then, an electrodepositable composition is complex in terms of the freedom or availability with respect to removal of a component or in terms of the apparent molecular size of a given vehicle component.

Apparatus for carrying out the present process comprises an electrodeposition bath in which the electrode upon which the coating is deposited is separated from the counterelectrode, at least in part, by a membrane, semipermeable or selecetively permeable, thus forming an electrodialysis compartment, dividing the coating zone from the counter-electrode. A selective separation unit is connected to the coating bath zone and has a physical barrier which passes aqueous efiiuent While retaining the solubilizing resin components. Means are provided for operating the selective separation unit continuously or intermittently to treat at least a portion of the contents from the coating zone. It is usually preferred that the electrodialysis membrane be a plant fiber or an ion-exchange membrane and that the compartment comprising the electrodialysis membrane and counter-electrode have a suitable means for flushing the compartment. The preferred selective separation unit is an ultrafiltration unit.

Illustrating such apparatus is FIG. 3 which is a schematic drawing depicting one embodiment of electrocoating apparatus suitable for use with the treating method herein described. Referring now to FIG. 3, a chemically-resistant tank 1 contains the aqueous coating composition. The electrode compartment 3, immersed in tank 1 and attached to tank 1 by fastening means 7 is fitted with a power source connector 5, input line 9 and output line 11, to facilitate flushing of electrode compartment 3 as hereinbefore described. The electrode in electrode compartment 3 is connected to a DC. power source 21 by means of electrical conductor 23. The power source 21 is also connected to bus bar 27 via conductor 25. The bus bar 27 contains electrical contact plate 29 which is employed to energize hanger 31. Articles 33, to be coated, are shown approaching tank 1, immersed in, and exiting and are in electrical connection with hanger 31. Hanger 31 is supported by grounded conveyor 37 and insulated from the energized contact plate 29 by means of insulator 35. In addition, tank 1 is connected to selective separator 13 via valve 15. The aqueous composition continuously or intermittently enters the selective separator 13 through valve 15, and upon processing the concentrated component is returned to tank 1 via line 17. The efiiuent from the selective separator is removed to waste or further processing via line 19.

As hereinabove described there may be several electrode compartments or, in some instances, it may be feasible to operate the tank periodically without employing membranes. In such a case, the electrode is simply removed from the compartment and thus installed in the bath without being enclosed by a membrane. The selective separator may also be operated when the electrodialysis apparatus is disengaged, simultaneously with the electrodialysis apparatus, when there is no coating being deposited, or while deposition is taking place but in the absence of electrodialysis.

The embodiment presented in FIG. 3 is not intended to be a complete description of the required piping, pumping, power sources and electrical circuitry required, but is only presented by way of illustration and is not to be construed as limiting the invention disclosed herein in any way.

The utilization of electrodialysis in combination with ultrafiltration in controlling and removing undesirable accumulated components in electrodepositable compositions is further described in conjunction with the following examples, which are to be considered illustrative rather than limiting. All parts and percentages in the examples and throughout the specification are by weight unless otherwise indicated.

In evaluating the electrodialysis and ultrafiltration combination, the following compositions were employed:

PASTE A Parts by weight 20 percent maleinized oil (total solids content 97.6 percent) 14.30 Diethylamine 2.08

20 percent maleic anhydride, 80 percent linseed oil maleinized oil having a viscosity of 100,000 centipoises.

Mixed 20 minutes in a closed container. There was then added:

Parts by weight Deionized water 32.00 Dispersing agent (combination oil-soluble sulfonate and non-ionic sulfactant-Witco 912) 1.48

Anthracite coal (pigmentary) 20.00

Basic lead silicate 8.00 Manganese dioxide 2.00 Strontium chromate 2.00

The above components were ground in a conventional Zirco mill to a 7% Hegman grind gauge reading.

Paste A was reduced as follows:

The vehicle resin employed in formulating Composition C (below) was comprised of a maleinized tall oil fatty acid-adipic acid ester of a styrene-ally] alcohol copolymer of 1100 molecular weight and a hydroxyl functionality of 5 comprising 38.5 percent of the copolymer, 55.5 percent tall oil fatty acids, and 6.0 percent maleic anhydride as a 100 percent solids vehicle having an intrinsic viscosity of 120,000 centipoises and an acid number of 40.6.

Composition C had the following characteristics:

Solids content (percent) 45.1 pH 9.2 Pigment-to-binder ratio 0.21/ 1.0

Composition C was reduced to produce Composition D (below).

COMPOSITION D Composition C 2930.0 Deionized water 8070.0

Composition D had the following properties:

pH 9.45 Conductivity, umhos/cm., F. 2300 Total solids content (percent) 12.2 Ash 1.15 MEQ grams total 6.40 MEQ/ 100 grams solids 52.5 Nitrogen content (percent) 0.24 CO (p.p.m.) Ethyl Cellosolve (percent) 0.42

1 Milliequivalents of base.

An electrodepositing apparatus which enabled the continual coating of coil stock was filled with 6600 parts of Composition D. The electrodepositing apparatus was fitted with two cathode compartments, each cathode compartment being separated from the anode compartment by a cloth membrane. In this instance, the canvas membrane employed comprised a plain weave (Shachi No. 1) linen membrane. The cathode compartment utilized 1380 parts of deionized water as a receiving solvent, into which a small amount of an electrolyte may be added which facilitates ease of starting the electrodialysis process by lowering the resistivity of the electrolyte. Such electrolytes may include amines and salts such as ammonium sulphate, sodium sulphate, soda ash, potassium hydroxide, and the like. After the electrodialysis process is effected, the diffusate may be periodically removed and replaced with fresh receiving solvent, or if desirable, the diffusate may be continually purged and replaced with fresh receiving solvent, which will prevent a buildup of deleterious components in the diffusate.

Composition D was continually deposited on aluminum coil stock (4 inches wide) and as the coating solids content was depleted, 220 parts of Composition C were added at the termination of every 4: cycle. By cycle it is meant that suiiicient bath coating solids have been deposited on the aluminum stock, which would have depleted the bath of its entire solids content had it not been for the additions after each Ms cycle. Also, after each A cycle approximately 600 parts of the ditfusate was exchanged with fresh receiving solvent.

The ditfusate collected at the end of one cycle had the following characteristics:

MEQ/ 100 grams total 4.41 PH 12.2 Conductivity, ,amhos/cm, 75 F. 7300 Potassium cation (parts), 10.9 gm. as KOH.

At the termination of one cycle, the dialyzate (the dialyzed coating Composition D) had the following characteristics:

pH 10.5 Conductivity, nmhos/cm, 75 F. 295

MEQ/ 100 grams total 7.96 MEQ/lOO grams solids 69.2 Solids content (percent) 11.5

The dialyzate (dialyzed coating Composition D) was then subjected to an ultrafiltration process which employed a Membrane B type at 50 p.s.i. The Membrane B had the characteristics as hereinabove described.

The entire composition was subjected to ultrafiltration whereby 3300 parts of ultrafiltrate were removed; dialyzate was again rediluted and subjected to ultrafiltration whereby an additional 2780 parts of ultrafiltrate were removed. The ultrafiltrate had the following characteristics:

During a second cycle Composition E was subjected to electrodialysis and ultrafiltration in a manner similar to that employed in Cycle 1 except that in the second cycle the ditfusate was continually removed and replaced at the rate of 100 cc. per minute with fresh receiving solvent.

The results of the first and second cycles are summarized in the following table:

TABLE I Dialyzed Dialyzed Dialyzed and U ultraultrafiltered filtered filtered product comco ce concenpletely rc- Dialyzed trate Dialyzed trate constituted 1st cycle composi- Ultrareconsti- 2nd cycle composi Ultrareconstiwith those destarting tion D Difiufiltrate tuted with starting tion E Difiufiltrate tuted with sirable compo- Oomposiafter one sate after after one deionized Composiafter 2nd sate for for 2nd deionized nents which tion D cycle one cycle cycle water tion E cycle 2nd cycle cycle water were removed Total parts by weight 6, 600 6, 395 5, 640 6,080 5, 940 6, 600 6, 964 16, 312 5, 537 6, 607 6, 600 pH 9. 45 10.15 12. 2 10. 0 9. 85 9. 9. 90 11. 9 9. 7 9. 7 9. 35 Conductivity 2, 300 2, 950 7, 300 1, 550 1, 580 2, 100 2, 880 2, 770 1, 450 1, 980 2, 265 Meq./liter 64. 0 79.6 44. 1 19. 3 59. 8 66.0 80.0 14. 4 18. 3 65. 2 65, 7 MeqJlOO Grams solids 52. 5 69. 2 59. 8 53. 7 64. 5 58. 2 54. 3 Total parts potassium as KOH 24.2 24.3 Solids content (percent) 12.1 Ash (percent) 1. Nitrogen content (percent) 0. 21 Total parts melamine. 62. 7 CO1 meqJliter 9. 20 Ethyl Cellosolve (percent) 0. 40 W.S.A. meq./1iter 2. 20

prnhos/cm. at 75 F.

pH 10.0 Conductivity, nrnhos/cm, 75 C. 1550 MEQ/100 grams total 1.93 Solids content (percent) 1.15 Nitrogen content (percent) 0.17 CO (p.p.m.) 160.0 Ethyl Cellosolve (percent) 0.49 Potassium hydroxide 6.6

After the concentrate was reconstituted with deionized water, the following characteristics were obtained:

pH 9.85 Conductivity, mhos at 75 F. 1580 MEQ/ 100 grams total 5.98 MEQ/100 grams solids 59.8 Solids content (percent) 10.0 Nitrogen content (percent) 0.14

Ethyl Cellosolve (percent) 0.14

The dialyzed concentrate, in preparation for a second cycle of electrodialysis and ultrafiltration, was reconstituted as follows:

In the following table some pertinent data and operating parameters are listed:

In Table III, that follows, the relative amount of potassium ion removed by the electrodeposition, electrodialysis and ultrafiltration processes are presented in tabular form.

Another advantage of apparatus used in this invention is that it provides for the accelerated removal of water from the coating zone. For example, it is especially useful to remove excess water which has been introduced into the coating zone by rinsing coated articles over the coating zone. By such rinsing over the coating zone, the drag-out material adhering to the electrocoated parts is returned to the coating zone, thus any solubilizing agent which would have been removed is returned to the coating zone. When the process is carried out in this manner, the electrodialysis and selective separation steps are operated for a time sufficient to remove both the excess water and the solubilizing agent which was returned to the coating zone.

Utilizing the electrodeposition apparatus and composition described above, but operating the process to return dragout to the bath, the proportion of potassium hydroxide removed would be as shown in Table IV.

TABLE IV First cycle, percent Second cycle,

percent No No drag- Dragdrag- Drag- In Table IV the data for the ultrafiltration step refleets an increased proportion of potassium hydroxide removed; the increase is limited to the ultrafiltration step in this instance because in each cycle, the electrodialysis and electrodeposition processes are carried out prior to subjecting the composition to ultrafiltration. Thus, it would be expected that a variation in percentage of Table IV would be obtained if the processes were performed simultaneously or in diiferent sequence.

It is important to note the potassium content after electrodialysis and ultrafiltration (see Table I). At the termination of electrodialysis, the potassium content increased from 23.7 to 28. 6 as grams of KOH; this increase can 'be readily explained as due to the electrodeposition process in that, as the resin is deposited on the anode, the potassium is released to accumulate in the bath, and also as new feed material is introduced into the bath, the concentration of potassium increases because potassium hydroxide has been utilized as a solubilizing agent in the said feed material.

The electrodialysis alone is not sufficient to maintain the potassium content at the initial concentration of Composition D. However, upon subjecting the dialyzed Composition D to ultrafiltration, the potassium content may be restored to an operable level. The second cycle 16 utilizing Composition E produced results similar to those obtained in the hereinabovedescribed first cycle. In production operation, ultrafiltration and electrodialysis may 'be used simultaneously, thus pH, conductivity, etc. would be relatively constant.

Another embodiment of this invention, utilizing an epoxy ester resin system is set forth below. Essentially the same electrodeposition apparatus, membranes, and ultra filtration unit as described in the previous embodiment were employed.

The resin utilized was a tall oil fatty acid-epoxy ester comprised of 45.08 percent Epon resin (Shell Chemicals Epon 829), 25.66 percent tall oil fatty acid, and 29.26 percent maleinized tall oil fatty acid. The resin had the following characteristics:

Solids content (percent) 80.6 Viscosity (centipoises) 68,000

The above components were ground in a conventional zircoa mill to a 7 A Hegman grind gauge reading.

Paste F was reduced as follows:

COMPOSITION G Paste F 136.80 Epoxy ester resin 313.70 Potassium hydroxide solution (15 percent in water) 31.78 Deionized water 400.34 Linseed oil fatty acids 5.50 Anti-cratering agent 1 2.96

1 Aminesolubillzed acrylic resin.

Composition G had the following characteristics:

Solids content (percent) 38.5 pH 9.0 Pigment-to-bindcr ratio .257/1 Pigment volume concentration (percent) 19.9

Composition G was reduced to produce Composition H as follows:

COMPOSITION H Parts by weight Composition B 892.42 Deionized water 2537.58

A total of 6600 parts of Composition H were charged into an electrocoating apparatus and the bath contents were turned over three times in a manner previously disclosed. After Composition H had been subjected to electrodialysis and ultrafiltration, it was reconstituted (Composition I) by replacing those minute essential components that were removed during the electrodialysis and ultrafiltration processes. Likewise, after the second cycle, Com position I was reconstituted (Composition J) in a similar manner.

As the bath was continually operating, the solids content was depleted and such depletion was restored at the end of each A; cycle with a sufiicient quantity of Composition G to restore the solids content to the initial level. of the total 1200 parts catholyte that were charged initially, 600 parts were replaced every cycle with fresh deionized water. The results of the three cycles are summarized in the following Table V.

t will be observed that electrodialy was insuflicient to maintain the bath parameters, for example d Again, i sis alone uring the electrodialysis process both the pH and conductivity of the bath continued to rise, and were not restored to normal until after at least a ion of port the bath was subjected to ultrafiltration.

In such compositions as described, in the absence of ultrafiltration, the potassium content in the bath would continue to rise, being accompanied by an increase in pH ffect on the deposited film thickness and other properties. The undesirable elfects of increased potassium content can now be removed or controlled by subjectin the bath to ultrafiltration.

and conductivity, which have a deleterious e Also, in instances where it has been sought to control the bath by the sole use of ultra'filtration, cases been unsuccessful due to the slow of the potassium cation through the particular ultrafiltration membrane. In such instances, it'has been necessa to employ electrodialysis in conjunction W1 filtration process in order to provide for sufficient control of the bath, thus maintaining uniform coating parameter and film properties.

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We claim:

1. In a method of electrocoating an electrically-conductive surface from an electrodeposition bath comprising ionically solubilized synthetic resin in aqueous medium, the steps comprising subjecting at least a portion of the electrodeposition bath to electrodialysis wherein ions of charge opposite to said resin are passed through a membrane under an electromotive force and removed from the electrodeposition bath and simultaneously, or at a separate time subjecting at least a portion of the electrodeposition bath to an ultrafiltration process wherein an ultra'sfiltration membrane passes water and solute of substantially lower molecular size than said resin, while retaining said resin and returning retentate of the ultrafiltration process to the electrodeposition bath.

2. A method as in claim 1 wherein the resin is a basesolubilized synthetic polycarboxylic acid.

3. A method as in claim 2 wherein the base is potassium hydroxide.

4. A method as in claim 1 wherein the resin is an acidsolubilized polybasic resin.

5. A method as in claim 1 wherein the electrodialysis is carried out by interposing a semi-permeable or selectively-permeable membrane between said surface being electrocoated and a counter electrode in the electrodeposition bath during the electrocoating process.

6. The method of claim 1 wherein at least a portion of an aqueous ultrafiltrate eflluent is utilized to rinse paint drag-out back into the electrocoating tank.

7. An electrocoated article obtained by the process of claim 1.

8. In an electrodeposition process which comprises passing an electric current through an aqueous bath containing ionically-solubilized synthetic resin in electrical contact between an article to be coated, serving as an electrode and a counter-electrode, the improvement which comprises carrying out the process with the article being coated and a counter-electrode being separated by an electrodialysis membrane in an electrodialysis compartment into which ions are passed through said membrane while continuously or intermittently subjecting at least a portion of said bath to an ultrafiltration process wherein an ultrafiltration membrane passes Water and solute of substantially lower molecular size than said resin, while retaining said resin, and returning retentate of the ultrafiltration process to said bath.

9. A method as in claim 8 wherein said electrodialysis membrane is an ion-exchange membrane.

'10. A method as in claim 8 wherein the electrodialysis membrane is a cation-exchange membrane having a pore size of less than 20A.

11. A method as in claim 8 wherein the electrodialysis membrane is a cloth of plant fibers.

12. A method as in claim 11 wherein the electrodialysis membrane is comprised of a cloth prepared by plain Weaving linen fibers.

13. A method as in claim 8 wherein a plurality of counter-electrodes are employed and a plurality of electrodialysis compartments are employed.

References Cited UNITED STATES PATENTS 3,419,480 1 2/1968 Cooke 2041'81 3,444,066 5/ 1969 Brewer et al. 204181 3,526,588 9/ 1970 Michaels et a1. 210-23 3,556,970 '1/1971 Wallaceetal 204-181 FOREIGN PATENTS 1,071,458 6/1967 Great Britain 2041 81 HOWARD S. WILLIAMS, Primary Examiner U.S. C1.X.R. 

